Area of the Art
The present invention concerns paper shredders and more particularly a paper shredder with an auto-feed mechanism with user replaceable parts.
Description of the Background of the Invention
Modern paper shredders for shredding documents incorporate counter rotating shafts bearing blades and spacers so that paper passing between the blades is effectively shredded. The maximum sheet capacity of such paper shredders is dependent on the strength of the motor and the limitations of the cutting blades and rotary shafts. Although many units can handle small stacks of 10-20 sheets, when a large amount of paper must be destroyed, the user needs to stand beside the unit and continuously feed the machine stacks of paper that are within the maximum sheet capacity of the unit. If even a relatively small number of sheets are to be shredded, it is usually necessary for the user to get up and walk to the shredder because most units must be fed relatively small quantities of paper at one time—that is, they cannot shred large stacks of paper.
The problem of conveniently shredding large stacks can be solved by paper shredders equipped with some type of an auto-feed mechanism that allows automatic feeding of the paper shredder from large stacks of documents. U.S. Pat. No. 5,362,002, the contents of which are incorporated herein by reference, discloses a paper shredder with such an automatic paper feeding device. The feeding device includes an angled tray which is mounted to the device's top adjacent the shredding roller assembly, a rotary shaft which is mounted rotatably on the tray, a tension spring which is connected to the rotary shaft and the tray, and at least one push rod, which has two pivotally connected rod sections. The device operates by lifting one or a few sheets of paper from the top of the paper stack in the tray and transferring them into the throat of the shredder. A similar arrangement is found in U.S. Pat. No. 5,884,855, also incorporated herein by reference, which discloses a paper feed structure for paper shredders having a paper containing tray and paper feed adjustment device. Both of these auto-feeding devices simulate manual feeding of paper into a shredder and both of them depend on an external tray which increases the overall height and profile of the unit.
What is really needed is a shredder with an integral auto-feed system. A good example is disclosed in U.S. Pat. No. 8,074,912, the content of which is incorporated herein by reference, wherein the paper shredder includes a driving roller assembly at the bottom of a paper compartment and a hinged top for closing the paper compartment and providing downward pressure on the stack of paper located within the compartment. The driving roller assembly includes a spaced apart pair of counter rotating feed rollers which extend through an opening in the bottom of the paper compartment to contact and grab a sheet of paper from the bottom of a stack of paper that has been placed in the compartment. The sheet is grabbed near its center and pulled between the rollers and then pushed into the shredding mechanism. When the compartment is empty, one or more sheets of paper can be pushed through a slot in the hinged top directly between the feeding rollers and into the shredding mechanism. Thus, this unit can function either as an auto-feed unit which shreds a large stack of sheets placed in the compartment or, when the compartment is empty, as a conventional paper shredder by inserting sheets directly into the shredding mechanism.
As long as the hinged top is designed to provide the proper amount of pressure to the stack, the bottom feeding roller pair works quite well. However, one problem with the feed rollers is that each roller has only a single point of contact with the bottom of the paper stack. If this contact proves inadequate to pull a sheet of paper from the bottom of the stack, the whole system fails. Such a failure occurs if the rollers become worn so that they can no longer “grab” the contacting sheet of paper. The same thing may happen if there is a “slippery” spot on the lower side of the bottom sheet of paper.